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Threads, SMP, and Microkernels Processes Resource ownership - process is allocated a virtual address space to hold the process image Dispatched - process is an execution path through one or more programs execution may be interleaved with other processes These two characteristics are treated independently by the operating system Processes Dispatching is referred to as a thread Resource of ownership is referred to as a process or task Multithreading Operating system supports multiple threads of execution within a single process MS-DOS supports a single thread UNIX supports multiple user processes but only supports one thread per process Windows NT, Solaris, Linux supports multiple threads Threads and Processes one process one thread multiple processes one thread per process one process multiple threads multiple processes multiple threads per process Processes Have a virtual address space which holds the process image Protected access to processors, other processes, files, and I/O resources Threads Has an execution state (running, ready, etc.) Saves thread context when not running Has an execution stack Has some per-thread static storage for local variables Has access to the memory and resources of its process all threads of a process share this Single Threaded and Multithreaded Process Models Multithreaded Process Model Single-Threaded Process Model Process Control Block User Address Space Thread Thread Thread Control Block Thread Control Block Thread Control Block Process Control Block User Stack User Stack User Stack User Address Space Kernel Stack Kernel Stack Kernel Stack User Stack Kernel Stack Thread Benefits of Threads Takes less time to create a new thread than a process Less time to terminate a thread than a process Less time to switch between two threads within the same process Since threads within the same process share memory and files, they can communicate with each other without invoking the kernel Threads Suspending a process involves suspending all threads of the process since all threads share the same address space Termination of a process, terminates all threads within the process Thread operations: spawn, block, unblock, finish (no suspend, only active) User-Level Threads All thread management is done by the application, and is fast The kernel is not aware of the existence of threads Thread switching does not require kernel mode privileges Scheduling is application specific Kernel-Level Threads Windows NT, Linux and OS/2 are examples of this approach Kernel maintains context information for the process and the threads Switching between threads requires the kernel and may be slower than user-level Combined Approaches for Threads Example is Solaris Thread creation done in the user space Bulk of scheduling and synchronization of threads done in the user space Relationship Between Threads and Processes Threads:Process Description Example Systems 1:1 Each thread of execution is a unique process with its own address space and resources. Most UNIX implementations M:1 A process defines an address space and dynamic resource ownership. Multiple threads may be created and executed within that process. Windows NT, Solaris, Linux, OS/2, OS/390, MACH Relationship Between Threads and Processes Threads:Process 1:M M:M Description Example Systems A thread may migrate from one process environment to another. This allows a thread to be easily moved among distinct systems. Ra (Clouds), Emerald Combines attributes of M:1 and 1:M cases TRIX Categories of Computer Systems Single Instruction Single Data (SISD) single processor executes a single instruction stream to operate on data stored in a single memory Single Instruction Multiple Data (SIMD) one instruction is executed on a different set of data by the different processors Categories of Computer Systems Multiple Instruction Single Data (MISD) a sequence of data is transmitted to a set of processors, each of which executes a different instruction sequence. Never implemented Multiple Instruction Multiple Data (MIMD) a set of processors simultaneously execute different instruction sequences on different data sets Parallel Processor Architecture MIMD Distributed memory (self-contained computers) Shared memory multiprocessor Clusters Master/Slave Symmetric multiprocessors (SMP) Symmetric Multiprocessing Kernel can execute on any processor Typically each processor does selfscheduling from the pool of available process or threads portions of the kernel can execute in parallel in different processors Symmetric Multiprocessor Organization Processor Cache Processor Cache Main Memory . . I/O Subsystem . Processor Cache Multiprocessor OS requirements Must provide functionality of a multiprogramming OS plus additional features to support multiple processors Simultaneous concurrent processes or threads: kernel routines need to be reentrant Scheduling done by any processor, can create conflicts. Synchronization through locks is required. Memory management needs to be coordinated in the different processors Much more complex than just multiprogramming OS. Microkernel Small operating system core Contains only essential operating systems functions Many services traditionally included in the operating system are now external subsystems device drivers file systems virtual memory manager windowing system and security services Benefits of a Microkernel Organization Uniform interface on request made by a process all services are provided by means of message passing Extensibility allows the addition of new services Flexibility existing features can be subtracted Benefits of a Microkernel Organization Portability changes needed to port the system to a new processor is changed in the microkernel - not in the other services Reliability modular design small microkernel can be rigorously tested Benefits of Microkernel Organization Distributed system support message are sent without knowing what the target machine is Object-oriented operating system components are objects with clearly defined interfaces that can be interconnected to form software Microkernel Design Primitive memory management mapping each virtual page to a physical page frame Inter-process communication I/O and interrupt management Windows NT Processes Implemented as objects An executable process may contain one or more threads Both process and thread objects have built-in synchronization capabilities Windows NT Process Object Attributes Process ID Security Descriptor Base priority Default processor affinity Quota limits Execution time I/O counters VM operation counters Exception/debugging ports Exit status Windows NT Thread Object Attributes Thread ID Thread context Dynamic priority Base priority Thread processor affinity Thread execution time Alert status Suspension count Impersonation token Termination port Thread exit status NT Thread Status Runnable Standby Pick to Run Preempted Ready Unblock/Resume Resource Available Resource Available Transition Waiting Unblock Resource Not Available Not Runnable Switch Running Block/ Suspend Terminate Terminated Solaris Process includes the user’s address space, stack, and process control block User-level threads Lightweight processes Kernel threads Solaris User Level Threads Stop Runnable Wakeup Continue Preempt Stop Stopped Dispatch Stop Sleep Active Sleeping Solaris Lightweight Processes Timeslice or Preempt Running Wakeup Dispatch Stop Stopped Runnable Blocking System Call Continue Wakeup Active Stop
